1. Field of the Invention
The present invention relates to a two-bladed wind turbine, and an associated method for controlling the yawing action of the wind turbine.
2. Description of Related Art
During wind turbine operation, the wind turbine structure is often exposed to various stresses and strains. The fatigue effects of such stresses must be accounted for during the wind turbine design process, to ensure that the wind turbine itself is able to withstand such strains without undergoing a catastrophic structural failure. This may involve designing wind turbines with considerable structural reinforcement, to prevent breakages or fractures of the structure due to such forces.
Accordingly, it is desirable that methods of operating wind turbines are developed that reduce the magnitude and/or effect of these operational stresses, thereby reducing the structural design requirements for a wind turbine, resulting in a corresponding saving in wind turbine manufacturing effort and cost.
One of the areas where a wind turbine experiences considerable forces is during the yawing of a wind turbine nacelle and rotor to face into the direction of the wind at the turbine. As a wind turbine is yawed, the wind turbine structure experiences a yaw moment due to the effect of yawing or turning the nacelle while the wind turbine blades are in motion. Such moments generate forces in the wind turbine blades, which are required to be dimensioned to withstand the effects of such forces. In addition, it is important that any yawing action is carried out in a fast and efficient manner, in order to reduce any energy losses due to the existence of a yaw error between the wind direction at the turbine and the direction that the wind turbine blades are currently facing.
A known solution to the problem of yaw forces is the use of teether hubs on wind turbines, which allow for the relative movement of the rotor hub on the wind turbine nacelle. However, the use of such teeter hubs on wind turbines involves considerable technical problems during the design stage, as well as increasingly complex service operations.
EP 2 314 869 describes a yaw control system for a wind turbine generator that minimizes the wear of the wind turbine during production and yawing by performing yaw rotation at an earlier stage of change of the wind direction. The yaw control system employs a set of conditions that detects transitional changes in the wind direction for time durations above one of two threshold values, thereby detecting changes faster than nominal wind direction measurements.